Trivalent cobalt in nickel-zinc ferrites—I. chemical problems

By using special preparation techniques the amounts of trivalent cobalt present in irondeficient Ni-Zn-Co ferrites have been varied.Co³⁺ concentrations have been measured very accurately by careful chemical analyses. It has been shown that under normal sintering conditions, e.g. 1200°C in oxygen, the amount of Co³⁺ just compensates the iron deficiency. The cation vacancy concentration is low. Cation vacancies are formed at lower temperatures near pores and grain boundaries.     

Trivalent cobalt in nickel-zinc ferrites—II. magnetic relaxations

In a number of Ni- and NiZn-ferrites containing Co²⁺- and Co³⁺-ions magnetic measurements of the initial permeability in the kHz range above room temperature reveal the presence of a relaxation process.From the measurements it can be concluded that Co²⁺ is the anisotropic ion responsible for the relaxation. The Co³⁺ ions and presumable cation vacancies in association with Co³⁺ provide the means whereby Co²⁺ is transported through the lattice.     

Scratch-modified magnetic domain patterns in garnet films

If a loaded Al₂O₃ sphere is passed over SmGa and LuTb garnet crystalline films, the domain pattern is modified. The width of the changed pattern is proportional to the diameter of the zone of contact between the sphere and the garnet. The pattern depends on load: a herring-bone type is found at low loads, a parallel pattern at high loads. After a magnetic field has disturbed the pattern, a narrow region of straight domains is left, which persists unless the material is heated above 1100°C. The patterns are independent of the sign of the magnetostriction constants.     

Elektrolytadsorption und Aktivitätskoeffizient,IV

Colloid and Polymer Science -     

Gradient Elasticity Based on Laplacians of Stress and Strain

Using Mindlin’s general microstructural elasticity theory we derive models pertaining to a gradient elasticity based on both Laplacians of stress and strain. We prove that such Laplacian-based gradient elasticity models can also be derived on the basis of a thermodynamically consistent gradient elasticity. The proof relies upon a non-conventional thermodynamic framework. It is shown that a general analogy between linear viscoelastic solids based on spring and dashpot elements and specific gradient elasticity models can be established. The governing differential equations of motion of resulting gradient elasticity models are then formulated. By employing energy related arguments, conditions on the material parameters are derived and the appropriate concomitant boundary conditions are specified for both approaches. Finally, the considered models are compared to each other with reference to one-dimensional dispersion relations.     

Nachwirkungsmechanismen in Ferriten

In ferrites a large number of after-effects are found, with time constants between nano-seconds and years. In this review the after-effects due to ion-and electron motion will be treated. One finds:

1. single-ion effects in combination with lattice deformations, e.g. Mn³⁺;
2. ion effects caused by mobile vacancies, e.g. Co²⁺;
3. effects due to electron transfer:
4. Co²⁺?Co³⁺
5. Me²⁺?Fe³⁺, in combination with Me⁴⁺ and vacancies.
6. Me⁴⁺?Fe²⁺, with Me=Si, Ti (photomagnetic effect).

The electron transfer is found to be related to electrical effects. In analogy to the photoelectric effect, one has found that illumination produces changes in magnetic properties. Generally speaking, one has in ferrites as many problems with donors and acceptors as in other semiconductors. Information from magnetic measurements helps to elucidate their nature.